docs(ToDo): Add link to added toc of ToDo-App guide

[DevTKSS]

GitHub Issue (If applicable): closes #

PR Type

What kind of change does this PR introduce?

• Documentation content changes

What is the current behavior?

Only Source link with captions, not really good to lookup for beginners

What is the new behavior?

Adding reference to the added docs of ToDo App in Samples Repository

PR Checklist

Please check if your PR fulfills the following requirements:

• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Validated PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
• Commits must be following the Conventional Commits specification.

Other information

Related to unoplatform/Uno.Samples#875
Merge Sample before this PR to successfully build DocFx!

PR in samples is first part and first enhances and providing the structure to proceed on docs creation like by markdownlint fixes and better toc structure with code-csharp tags to enable visibility of the code without having to visit sourcecode actually

Internal Issue (If applicable):